Dci.php

<?php
//...
class Dci
{  
    /**
     * @Groups("dcis:read", "dcis:write", "medicaments:write")
     * @ORM\ManyToMany(targetEntity=Medicament::class, inversedBy="dcis", cascade={"persist"})
     *
    */
    private $dci;
}

Medicament.php

<?php
//...
class Medicament
{
     /**
     * @Groups("medicaments:read", "medicaments:write", "dcis:write")
     * @ORM\ManyToMany(targetEntity=Dci::class, mappedBy="dci", cascade={"persist"})
     */
    private $dcis;
}

MedicamentController.php

<?php
//...
class ApiController extends AbstractController
{
  //...
  public function create(Request $request): JsonResponse
  {
      $object = $request->request->get('object');
      $context = [
        AbstractNormalizer::GROUPS => ['medicaments:write']
      ];
      $medicament = $this->serializer->denormalize($object, Medicament::class, 'json', $context);

      return new JsonResponse(
          $this->serializer->normalize($result),
          Response::HTTP_CREATED
      );
  }
}